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The axon is the single long process that carries a neuron’s output away from the cell body, and the axon hillock is the region where the axon emerges from the soma. In recordings from layer 5 pyramidal neurons, the action potential starts about 35 micrometres beyond the hillock, in the axon initial segment, rather than in the hillock itself.
Short answer
The hillock is the base of the axon at the soma. The axon initial segment (AIS) is the stretch of axon that begins there and runs to the start of myelin. The AIS, not the hillock proper, is where the threshold for firing is lowest and where most spikes begin.
Axon and axon hillock: definitions and scope
An axon is a thin, non-spiny process that leaves the base of the soma and heads toward the white matter (Palmer and Stuart, 2006). The axon hillock is the point where it emerges from the cell body. Clark, Goldberg and Rudy (2009) define the axon initial segment as the proximal portion of the axon beginning at that emergence point, the hillock, and ending where myelin starts.
So the two terms describe neighbouring regions, not synonyms: the hillock is where the axon leaves the soma, and the AIS is the specialised axon segment that follows. For broader background on the field, see the guides on neurobiology and neuroscience.
Where the action potential starts
Clark and colleagues (2009) report that action potential initiation occurs at the AIS because the threshold for generating a spike is lowest there, and that this is likely true for most neurons. Palmer and Stuart (2006) measured how far from the soma that site sits, and Clark and colleagues (2009) describe how the AIS is divided.
- In layer 5 pyramidal neurons in rat brain slices, the shortest onset latency was found at 35.6 ± 2.3 micrometres from the axon hillock. The AIS in these cells is about 40 micrometres long, and the first node of Ranvier lies about 90 micrometres from the soma (Palmer and Stuart, 2006).
- Palmer and Stuart report that the initiation site was robust, including under axonal inhibition, and was at a similar distance in neurons from 5-day-old animals as in mature ones (Palmer and Stuart, 2006).
- Clark and colleagues (2009), citing Kole and others, divide the AIS into a proximal portion (5 to 30 micrometres from the hillock) and a distal portion (35 to 55 micrometres), and report that Kv1 current is greatest in the distal portion.
The same spike, shaped as it travels
Action potential half-width falls from roughly 500 microseconds at the soma to roughly 250 microseconds at the distal AIS (Clark et al., 2009). The AIS is therefore not only a trigger point but a site where the waveform is tuned by the channels packed into it.
Parts compared: what sets the initiation site
| Structure or component | Role described in the sources | Source |
|---|---|---|
| Axon hillock | Where the axon emerges from the soma; the AIS begins here | Clark et al., 2009 |
| Axon initial segment | Lowest spike threshold; usual site of action potential initiation | Clark et al., 2009 |
| Nav1.6 sodium channels | The most abundantly expressed sodium channel isoform in the nervous system; prevalent at the AIS | Clark et al., 2009 |
| Nav1.1 sodium channels | Found mainly at the AIS of fast-spiking GABAergic interneurons | Clark et al., 2009 |
| Ankyrin G | Required to cluster sodium and KCNQ2/3 channels at the AIS; when it is knocked down the axon takes on dendritic properties | Clark et al., 2009 |
| First node of Ranvier | About 90 micrometres from the soma in layer 5 pyramidal neurons | Palmer and Stuart, 2006 |
Where it matters for labs and recording work
Distance from the axon hillock is the reference used to report where a spike begins, which is why the 35.6 micrometre figure above is stated that way. Palmer and Stuart (2006) note that the axon’s small diameter and myelination make direct recording difficult. Related method guides on two-photon microscopy, confocal microscopy and immunofluorescence controls cover imaging approaches that apply to cell compartments like these.
Common misunderstandings
Misunderstanding 1: the hillock is where the spike fires
The hillock is the point where the axon leaves the soma, but the measured threshold and latency data above place initiation in the AIS, tens of micrometres beyond the hillock in layer 5 pyramidal neurons.
Misunderstanding 2: every neuron is the same
The figures here come from layer 5 pyramidal neurons in rat slices, and Clark and colleagues (2009) describe different sodium channel isoforms in different cell types, for example Nav1.1 mainly in fast-spiking GABAergic interneurons. Treat the micrometre values as specific to that cell type. How densely sodium channels are packed in the AIS is also still debated in the literature (Palmer and Stuart, 2006).
FAQ
What is the axon hillock?
It is the region where the axon leaves the cell body, and the point at which the axon initial segment begins.
What is a neuron axon?
A neuron axon is a nonspiny process that originates at the base of the soma and projects toward the white matter in the layer 5 pyramidal neurons studied by Palmer and Stuart (2006). It carries the cell’s output away from the cell body.
What is the difference between the axon and the axon hillock?
The axon is the whole output process of the neuron. The hillock is only its origin at the soma.
Where does the action potential start in a neuron axon?
Mostly in the axon initial segment, where the spike threshold is lowest. In layer 5 pyramidal neurons it was found about 35 micrometres from the hillock.
What proteins organise the initial segment?
Ankyrin G is required to cluster sodium channels and KCNQ2/3 channels there, and Nav1.6 is prevalent at the AIS.
Sources
- Clark BD, Goldberg EM, Rudy B. Electrogenic tuning of the axon initial segment. Neuroscientist. 2009;15(6):651-668. https://pmc.ncbi.nlm.nih.gov/articles/PMC2951114 (checked 2026-10-29)
- Palmer LM, Stuart GJ. Site of action potential initiation in layer 5 pyramidal neurons. J Neurosci. 2006;26(6):1854-1863. doi:10.1523/JNEUROSCI.4812-05.2006. https://pmc.ncbi.nlm.nih.gov/articles/PMC6793621/ (checked 2026-10-29)
